Lithographic apparatus, illumination system, filter system and method for cooling a support of such a filter system

ABSTRACT

A lithographic apparatus includes an illumination system configured to condition a radiation beam, a projection system configured to project the radiation beam onto a substrate, and a filter system for filtering debris particles out of the radiation beam. The filter system includes a plurality of foils for trapping the debris particles, a support for holding the plurality of foils, and a cooling system having a surface that is arranged to be cooled. The cooling system and the support are positioned with respect to each other such that a gap is formed between the surface of the cooling system and the support. The cooling system is further arranged to inject gas into the gap.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of priority from U.S.Provisional Patent Application No. 60/639,774, filed Dec. 29, 2004, theentire content of which is incorporated herein by reference.

FIELD

The present invention relates to a lithographic apparatus, anillumination system, a filter system, and a method for cooling a supportof such a filter system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction), while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a lithographic apparatus, the size of features that can be imagedonto the substrate is limited by the wavelength of the projectionradiation. To produce integrated circuits with a higher density ofdevices, and hence higher operating speeds, it is desirable to imagesmaller features. While most current lithographic projection apparatusemploy ultraviolet light generated by mercury lamps or excimer lasers,it has been proposed to use shorter wavelength radiation, in the rangeof 5 to 20 nm, in particular around 13 nm.

Such radiation is termed extreme ultra violet (EUV) or soft X-ray andpossible sources include, for example, laser produced plasma sources,discharge plasma sources, or synchrotron radiation from electron storagerings. These types of radiation require that the beam path in theapparatus be evacuated to avoid beam scatter and absorption. Becausethere is no known material suitable for making a refractive opticalelement for EUV radiation, EUV lithographic apparatus must use mirrorsin the radiation (illumination) and projection systems. Even multilayermirrors for EUV radiation have relatively low reflectivities and arehighly susceptible to contamination, further reducing therereflectivities and hence throughput of the apparatus. This may imposefurther specifications on the vacuum level to be maintained and maynecessitate especially that hydrocarbon partial pressures be kept verylow.

In a typical discharge plasma source, plasma is formed by an electricaldischarge. The plasma may then be caused to compress so that it becomeshighly ionized and reaches a very high temperature, thereby causing theemission of EUV radiation. The material used to produce the EUVradiation is typically xenon or lithium vapor, although other gases,such as krypton or tin or water, may also be used. However, these gasesmay have a relatively high absorption of radiation within the EUV rangeand/or be damaging to optics further downstream of the projection beamand their presence should therefore be minimized in the remainder of thelithographic apparatus. A discharge plasma source is disclosed, forexample, in U.S. Pat. Nos. 5,023,897 and 5,504,795, both of which areincorporated herein by reference.

In a laser produced plasma source, a jet of, for example, (clustered)xenon may be ejected from a nozzle, for example, produced from anink-jet like nozzle as droplets or thin wire. At some distance from thenozzle, the jet is irradiated with a laser pulse of a suitablewavelength for creating a plasma that subsequently will radiate EUVradiation. Other materials, such as water droplets, ice particles,lithium or tin, etc. may also be ejected from a nozzle and be used forEUV generation. In an alternative laser-produced plasma source, anextended solid (or liquid) material is irradiated to create a plasma forEUV radiation. Laser produced plasma sources are, for example, disclosedin U.S. Pat. Nos. 5,459,771, 4,872,189, and 5,577,092, all of which areincorporated herein by reference.

During generation of EUV radiation, particles are released. Theseparticles, hereinafter referred to as debris particles, include ions,atoms, molecules, and small droplets. These particles should be filteredout of the EUV radiation, as these particles may be detrimental to theperformance and/or the lifetime of the lithographic apparatus, inparticular the illumination and projection system thereof.

International Patent Application Publication No. WO 99/42904,incorporated herein by reference, discloses a filter that is, in use,situated in a path along which the radiation propagates away from thesource. The filter may thus be placed between the radiation source and,for example, the illumination system. The filter includes a plurality offoils or plates that, in use, trap debris particles, such as atoms andmicroparticles. Also, clusters of such microparticles may be trapped bythese foils or plates. These foils or plates are orientated such thatthe radiation can still propagate through the filter. The plates may beflat or conical and may be arranged radially around the radiationsource. The source, the filter and the projection system may be arrangedin a buffer gas, for example, krypton, whose pressure is about 0.5 torr.Contaminant particles then take on the temperature of the buffer gas,for example, room temperature, thereby sufficiently reducing theparticles velocity before the end of the filter. This enhances thelikelihood that the particles are trapped by the foils. The pressure inthis known contaminant trap is about equal to that of its environment,when such a buffer gas is applied.

International Patent Application Publication No. WO 03/034153,incorporated herein by reference, discloses a contaminant trap thatincludes a first set of foils and a second set of foils, such thatradiation leaving the source first passes the first set of foils andthen the second set of foils. The plates, or foils, of the first andsecond set define a first set of channels and a second set of channels,respectively. The two sets of channels are spaced apart, leaving betweenthem a space into which flushing gas is supplied by a gas supply. Anexhaust system may be provided to remove gas from the contaminant trap.The pressure of the gas and the space between the two sets of channelsmay be relatively high so that debris particles are efficiently sloweddown, further enhancing the likelihood that debris particles are trappedby the second set of foils. The first and second set of channels providea resistance to the gas when the gas moves from the space between thetwo sets of channels in the channels of either the first or the secondset. Hence, the presence of the gas is more or less confined to thespace between the two sets of channels.

Even though the platelets or foils are positioned such that radiationdiverging from the radiation source can easily pass through thecontaminant trap, the foils or platelets do absorb some EUV radiationand, therefore, some heat. Moreover, these foils are heated by collidingdebris particles. This may result in a significant heating of the foilsand heating of a supporting structure that supports the foils. This maylead to thermal expansion of the foils and of the supporting structure.As optical transmission of the contaminant trap is very important in alithographic apparatus, the deformation of a foil due to thermalexpansion of the foil should be minimized.

European Patent Application Publication No. EP 1 434 098 addresses thisproblem by providing a contamination barrier, i.e. a foil trap orcontaminant trap, that includes an inner ring and an outer ring in whicheach of the foils or plates is slidably positioned at at least one ofits outer ends in grooves of at least one of the inner ring and outerring. By slidably positioning one of the outer ends of the foils orplates, the foils or plates can expand in a radial direction without theappearance of mechanical tension, and thus without thermally induceddeformation of the plate or foil. The contamination trap may includecooling means arranged to cool one of the rings to which the plate orfoils are thermally connected.

SUMMARY

It is desirable to provide a lithographic apparatus having a filtersystem, or an illumination system having a filter system, or a filtersystem itself, in which the foil trap can both be rotated, in order toactively intercept debris particles, and be cooled.

According to an aspect of the invention, there is provided alithographic apparatus that includes an illumination system configuredto condition a radiation beam, a projection system configured to projectthe radiation beam onto a substrate, and a filter system for filteringdebris particles out of the radiation beam. The filter system includes aplurality of foils for trapping the debris particles, a support forholding the plurality of foils, and a cooling system that has a surfacethat is arranged to be cooled. The cooling system and the support arepositioned with respect to each other such that a gap is formed betweenthe surface of the cooling system and the support. The cooling system isfurther arranged to inject gas into the gap.

According to an aspect of the invention, there is provided anillumination system configured to condition a radiation beam. Theillumination system includes a filter system for filtering debrisparticles out of the radiation beam. The filter system includes aplurality of foils for trapping the debris particles, a support forholding the plurality of foils, and a cooling system that has a surfacethat is arranged to be cooled. The cooling system and the support arepositioned with respect to each other such that a gap is formed betweenthe surface of the cooling system and the support. The cooling system isfurther arranged to inject gas into the gap.

According to an aspect of the invention, there is provided a filtersystem for filtering debris particles out of a radiation beam that isusable for lithography, in particular EUV lithography. The filter systemincludes a plurality of foils for trapping debris particles, a supportfor holding the plurality of foils, and a cooling system that has asurface that is arranged to be cooled. The cooling system and thesupport are positioned with respect to each other such that a gap isformed between the surface of the cooling system and the support. Thecooling system is further arranged to inject gas into the gap.

According to an aspect of the invention, there is provided a method forcooling at least a support for foils of a filter system for filteringdebris particles out of a radiation beam that is usable for lithography,in particular EUV lithography. The method includes positioning a cooledsurface with respect to the support such that a gap is formed betweenthe cooled surface and the support, and injecting gas into the gap.

As according to each of the above-mentioned aspects of the invention, agap is provided and a gas is used to transfer heat from the support tothe cooling system, the support can rotate while the cooling systemremains stationary. As the gas is injected into the gap, the gasexperiences much resistance in its movement from the confinement of thegap. The gas will only slowly leak into the surroundings and doestherefore not result in a sharp increase in the pressure of thesurroundings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 3 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 4 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 5 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 6 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 7 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 8 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a part of a filter system according to anembodiment of the invention;

FIG. 9 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a part of a filter system according to anembodiment of the invention;

FIG. 10 schematically depicts as a part of a lithographic apparatus, andof an illumination system, a filter system according to an embodiment ofthe invention;

FIG. 11 schematically depicts as a part of a lithographic apparatus, andof an illumination system, and of a filter system according to anembodiment of the invention; and

FIG. 12 schematically depicts the part shown in FIG. 11 seen from thepredetermined position that is intended to coincide with the source.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes: an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation, EUV radiation, or X-ray radiation); a support structure(e.g. a mask table) MT constructed to support a patterning device (e.g.a mask) MA and connected to a first positioner PM configured toaccurately position the patterning device in accordance with certainparameters; a substrate table (e.g. a wafer table) WT constructed tohold a substrate (e.g. a resist-coated wafer) W and connected to asecond positioner PW configured to accurately position the substrate inaccordance with certain parameters; and a projection system (e.g. arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g. including one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as, for example, whether or notthe patterning device is held in a vacuum environment. The supportstructure can use mechanical, vacuum, electrostatic or other clampingtechniques to hold the patterning device. The support structure may be aframe or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example, with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” as used herein should be broadlyinterpreted as referring to any device that can be used to impart aradiation beam with a pattern in its cross-section such as to create apattern in a target portion of the substrate. It should be noted thatthe pattern imparted to the radiation beam may not exactly correspond tothe desired pattern in the target portion of the substrate, for example,if the pattern includes phase-shifting features or so called assistfeatures. Generally, the pattern imparted to the radiation beam willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” as used herein should be broadlyinterpreted as encompassing any type of projection system, includingrefractive, reflective, catadioptric, magnetic, electromagnetic andelectrostatic optical systems, or any combination thereof, asappropriate, for the exposure radiation being used, or for other factorssuch as the use of an immersion liquid or the use of a vacuum. Any useof the term “projection lens” herein may be considered as synonymouswith the more general term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines, the additional tables may be used inparallel, or preparatory steps may be carried out on one or more tableswhile one or more other tables are being used for exposure.

The lithographic apparatus may also be of a type in which at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. In a path along which radiation propagates from thesource SO towards the illumination IL, a filter system FS is provided.The filter system FS substantially transmits the radiation and filtersdebris particles out of the radiation. The illuminator IL and the filtersystem may be regarded as at least a part of an illumination system. Thesource and the lithographic apparatus may together be separate entities,for example, when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system including, for example, suitabledirecting mirrors and/or a beam expander. In other cases the source maybe an integral part of the lithographic apparatus, for example, when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system if required, may be referred to as aradiation system.

The illuminator IL may include an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator and a condenser.The illuminator may be used to condition the radiation beam, to have adesired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF2 (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor IF1 can be used to accurately position themask MA with respect to the path of the radiation beam B, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the mask table MT may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) themask table MT may be connected to a short-stroke actuator only, or maybe fixed. Mask MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks). Similarly, in situations inwhich more than one die is provided on the mask MA, the mask alignmentmarks may be located between the dies.

The depicted apparatus may be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 2 schematically depicts a part of a lithographic apparatus, anillumination system, and a filter system according to an embodiment ofthe invention. Foil F1 and foil F2 are part of a filter system fortrapping debris particles. The filter system also includes a support, inFIG. 2 shown as part S1 and part S2. It is possible that part S1 andpart S2 both belong to one ring-shaped support. FIG. 2 may be seen asshowing a schematic cross-section of such a ring-shaped support. Asymmetry axis SA is schematically represented by line L. Foil F1 andfoil F2 may both be connected to an axis of the support. In that case,this axis may coincide with line L. However, it is also possible thatfoil F1 and foil F2 are not connected to each other, as will be shownlater. In an embodiment where the support, including support parts S1and S2, is ring-shaped, symmetry axis SA may coincide with a virtualstraight line that extends through a predetermined position that isintended to coincide with a source from which radiation is generated.

It is further possible that foil F1 and foil F2 are connected, i.e. formtogether one foil. It is in such an embodiment possible that support S1and support S2 are separated supports, separated by the foil. Forexample, support S1 may represent a cross-section of an outer ring,while support S2 represents a cross-section of an inner ring. In thatsituation, line L does not represent a symmetry-axis.

It is also possible that line L represents a plane of symmetry and thatthe filter system includes a plurality of foils that are parallel toeach other.

The filter system includes a cooling system CS. This cooling system CSmay include parts CS1 and CS2.

In cases where the respective support is ring-shaped, the respectivecooling system CS may also be ring-shaped. Line L may in someembodiments thus also represent the symmetry axis of the cooling systemCS. For a further description of the part of the filter system shown inFIG. 2, reference is only made to the upper part, i.e. above line L. Thedescription of the upper part also holds for the lower part.

The cooling system CS1 has a surface A1 that is arranged to be cooled.The cooling system CS1 and the support S1 are positioned with respect toeach other such that a gap G is formed between the surface A1 of thecooling system CS1 and the support S1. The cooling system CS1 is furtherarranged to inject gas into the gap G. The gas and its flow direction isindicated by dotted arrows. The path P between an entrance position EPat which the gas enters the gap G and an exit position XP from which thegas exits the gap G forms in the embodiment shown in FIG. 2 a meanderingpath P. As the path P is a meandering path, gas injected into the gapexperiences a large resistance when flowing from the entrance positionEP towards the exit position XP. Such a meandering path providesresistance to leakage of gas from the gap G towards its surrounding. Itis also possible that the path is a straight path. The resistanceexperienced by the gas when moving towards exit position XP is thenlower, as compared to the embodiment shown. The support S1 may beprovided with a recess R1 for holding the gas before the gas exit gap G.The pressure in this recess may be about 1000 Pa, whereas the pressureof the surrounding may be about 10 Pa. The recess R1 may thus provide abuffer in which injected gas cools the support S1.

The gap G may be such that a smallest distance between the surface A1and the support S1 is in a range that varies from about 20 micrometersto about 200 micrometers. The gap may also be such that the smallestdistance between the surface A1 and the support S1 is in a range thatvaries from about 40 micrometers to about 100 micrometers.

The surface A1 of the cooling system CS1 is arranged to be cooled with afluid. For this purpose, the cooling system CS1 may include a channelthat extends in a subsurface of surface A1. In use, water, that is,relatively cool water, may enter channel entrance CEA and run throughthe channel C, and leave the channel at channel exit CX. In that case,the subsurface of surface A1 will be cooled with water still about ascool as the water that enters the channel C at channel entrance CEA. Thecooling system CS1 may also be arranged to cool the gas before injectingthe gas into the gap G. Instead of having an entrance for water at aposition indicated by channel entrance CEA, it may be advantageous tolet water into the channel at a position indicated by CEG, so that waterfirst runs along an injection channel IC through which in use gas isinjected into the gap G. This allows for cooling the gas in theinjection channel IC or for further keeping the gas cool in theinjection channel in cases where the gas has been cooled before enteringthe injection channel IC. It is, of course, also possible that theinjection channel IC and the surface A1 are cooled by independentcooling mechanisms. Instead of using water, any other suitable coolingmedium may be used. Although not shown, it will be clear that entrancesand exits of channel C are connected with supplies and exhausts,respectively, such that no water and/or any other cooling medium usedfor cooling the cooling system will enter the surroundings of thecooling system and/or the filter system. Gas injected via injectionchannel IC into the gap G may be Argon, or any other gas that has goodcooling properties and is relatively inert.

When the filter system is exposed to EUV radiation and filters debrisparticles out of the path along which the EUV radiation propagatestowards a collection system, and the filter system rotates at about3,000 rpm in a vacuum environment, the foils and their support(s) arelikely to absorb about 1 kW of power as a result of absorption of EUVradiation, and impact of debris particles on foils. Without wishing tobe bound by any theory, it is indicated that it is possible to remove anamount of heat equal to about 1.3 kW when Argon gas is injected into thegap G such that a pressure of about 1000 Pa in the recess R1 is reached,the temperature difference between the support and the cooled surface ofthe cooling system CS1 is about 200 K, and the surface A1 includes anarea of about 1.26*10⁻² m². The heat transfer coefficient in thisconnection is taken to be about 0.7 W/m²*K*Pa and the efficiency isassumed to be about 0.85. The shortest distance between the support S1and the surface A1 in the gap is assumed to be between about 40 andabout 100 micrometers. Pressure in the surroundings may in that case beabout 10 mbar. In this assessment, the material of which the support ismade, is assumed to be stainless steel having a thickness of about 2 cmand a diameter of about 200 mm.

FIG. 3 depicts another part of a lithographic apparatus, illuminationsystem, and filter system according to an embodiment of the invention.In this situation, the support S1 and S2 include parts of a ring-shapedsupport that is rotatably arranged around a symmetry axis SA and acooling system CS, which may, in use, remain stationary with respect tothe support of which parts S1 and S2 are schematically shown. The foilsF1, F2 extend radially with respect to the symmetry axis SA. There maybe one injection channel IC, splitting in a part leading towards recessR1 and a part leading towards recess R2. Further structural features arethe same as depicted in FIG. 2. The cooling system CS shown in FIG. 3works the same as the cooling system shown in FIG. 2. It is possiblethat the support S1, S2 is rotatable due to a driving mechanism thattransmits forces towards an outer ring (not shown) to which the foilsF1, F2 in such an embodiment are connected. However, it also possiblethat the support S1, S2 are actually connected to cooling system CS via,for example, thermally insulating connections, and that the coolingsystem CS drives rotation of the support S1, S2. In this latterembodiment, it is not necessarily the case that the foils F1, F2 areconnected to, for example, an outer ring.

The combinations of a cooling system CS1 and support S1, (as well as S2and SR), may form at least a part of a heat sink HS1 (as well as HS2).Such a heat sink may be used in embodiments shown by FIGS. 4-12.

FIG. 4 schematically depicts a filter system FS which is arranged tofilter, in use, debris particles out of a predetermined cross-section ofthe radiation as emitted by a source (not shown). The filter system FSof FIG. 4 is shown as viewed from a predetermined position that is, inuse, intended to coincide with a position from which the sourcegenerates radiation. The foils F1, F2 are represented by lines for areason, which will become clear later on. In this example, thepredetermined cross-section is a section of the filter system FS asextending between a part referred to as S1 and a part referred to as S2.In this example, the predetermined cross-sections is thus asubstantially ring-shaped cross-section. The filter system FS includes afirst set of foils F1 and a second set of foils F2 for trapping thedebris particles. Each foil F1 of the first set is thermally connectedto a support S1, in this case, a first ring FR. Each foil F2 of thesecond set is thermally connected to a support S2, in this case, secondring SR. The first ring FR and the second ring SR are spatiallyseparated and have the same axis RA. Each foil F1 of the first setextends towards the second ring SR and each foil F2 of the second setextends towards the first ring FR. In other words, the support S1 may bering-shaped, and the support S2 may also be ring-shaped. Each foil F1 ofthe first set of foils F1 is, for example, soldered to the support S1.Each foil F2 of the second set of foils F2 is, for example, soldered tothe support S2. The foils F1, F2 may be made of a material includingsubstantially molybdenum. The supports may also be made of a materialincluding substantially molybdenum.

The filter system FS further includes a first heat sink HS1 and a secondheat sink HS2. Each foil F1 of the first set is thermally connected tothe first heat sink HS1 and each foil F2 of the second set is thermallyconnected to the second heat sink HS2. In use, through each foil F1 ofthe first set, heat is conducted towards substantially the first heatsink HS1. Through each foil F2 of the second set, heat is conductedtowards substantially the second heat sink HS2. The first set of foilsF1 extends substantially in a first section of the predeterminedcross-section, and the second set of foils F2 extends substantially in asecond section of the predetermined cross-section. The first sectionincludes all foils F1 of the first set and the second section includesall foils F2 of the second set. The first section and the second sectionare substantially non-overlapping.

It can be seen in FIG. 4 that the foils F1, F2 may be much shorter ascompared to a situation in which each foil would extend from support S1towards support S2. The amount of heat per foil F1, F2 to be transferredtowards the respective heat sink is much less as compared to the amountof heat that would have to be transferred to a heat sink in a situationwhere a foil were to be connected to one of the supports S1 or S2 andwhere the foil were to extend over the full distance between the supportS1 and the support S2 to which it may or may not be connected.

For structural strength of the foils F1, F2 and/or for spacing the foilsF1, F2 equally, substantially thermally insulating and relatively stiffwires may connect the free ends of the foils F1, F2. For the sake ofclarity these wires are not shown in any of the Figures.

It is also possible, as shown in FIGS. 4-7, that the foils F1, F2 of thefirst set and the second set, respectively, are apart from theirconnection with respective heat sink HS1, HS2, and unconnected withrespect to any other part of the filter system FS. This allows for agood optical transmission of the filter system FS, as well as a singlepath per foil for conducting heat away from the respective foil. Therelative dimensions of the first and second section, as well as therelative dimensions of the first foils F1 and the second foils F2, maybe chosen such that all of the filter system FS remains, in use, below apredetermined maximum temperature when, in use, exposed to the radiationbeam. Also, the cooling power of the respective heat sinks HS1, HS2 maybe chosen such that all of the filter system FS remains below apredetermined maximum temperature when exposed to the radiation beam. Ingeneral, the filter system FS may thus be arranged such that all of thefilter system FS remains below that predetermined maximum temperature.

As shown in FIG. 4, one foil F1 of the first set and one foil F2 of thesecond set extend in substantially the same virtual plane. A distance inthat virtual plane between the foil F1 of the first set and the foil F2of the second set is selected so as to maintain a gap between the foilF1 of the first set and the foil F2 of the second set when, in use, thefoil F1 of the first set and the foil F2 of the second set reach theirrespective maximum temperatures. This means that when, for each foil,their maximum thermal expansion is reached, the foils within one virtualplane will still not thermally connect. Each foil F1, F2 coincides witha virtual plane that extends through the predetermined position which isin use intended to coincide with a source (not shown) from which theradiation is generated. Hence, the foils F1, F2 are represented by linesin FIGS. 4-7. In use, the radiation will propagate along the foils F1,F2.

As the foils F1, F2 of a filter system FS according to an embodiment ofthe invention will remain below a predetermined maximum temperature whenexposed to, for example, EUV radiation, it is possible to design thefilter system such that the predetermined temperature is below thetemperature at which, for example, tin droplets, formed by tin debrisparticles, will not evaporate away from the foils F1, F2 when the foilis heated up.

FIG. 5 shows an embodiment of a filter system according to the inventionin which a foil F1 of the first set extends for a relatively small partbetween two foils F2 of the second set, and vice versa. This may havethe advantage that a sudden peak in optical transmission due to the gappresent between the foils F1 of the first set and the foils F2 of thesecond set, as will occur in the embodiment shown in FIG. 4, will notoccur in the embodiment shown in FIG. 5. Furthermore, if the filtersystem FS were to be rotated around a rotation axis RA, there would notbe an angular section present in the predetermined cross-section throughwhich debris particles may move along a direction into which theradiation propagates without being intercepted by the foils F1, F2. Itis possible to apply a number of second foils F2 that is smaller thanthe number of applied first foils F1, so as to allow a similar distancebetween all the foils F1, F2.

FIG. 6 shows an embodiment of a filter system FS according to theinvention in which a gap remains possible between a first foil F1 and asecond foil F2, which both extend within the same virtual plane.However, as the lengths of the foils F1, and the lengths of the foils F2alternate in a tangential direction, each gap between a first foil F1and a second foil F2 is “covered” by either a first foil F1 or a secondfoil F2 when the filter system rotates around the rotation axis RA.

FIG. 7 shows an embodiment of a filter system FS according to theinvention in which both the first set of foils F1 and the second set offoils F2 are more or less randomly distributed over, respectively, thefirst section and the second section of the predetermined cross-section.This may have the advantage that a possible inhomogeneity in the opticaltransmission of the filter system FS is more or less spread out over theentire predetermined cross-section. In other words, some peaks inoptical transmission may still occur, but the relative height is muchlower. As indicated before, at least a part of the filter system FS maybe movable such that each foil F1 of the first set and/or each foil F2of the second set may, in use, catch debris particles actively byintercepting debris particles in their course along a path along whichthe radiation propagates.

FIG. 8 and FIG. 9 both show a part of an embodiment of a filter systemaccording to the invention. FIG. 8 and FIG. 9 may both be regarded as aside view, in relation to FIGS. 4-7. The first support S1, in this casethe first ring FR, is shown to have a conical shape. The second supportS2, in this case the second ring SR, is shown to be cylindrical inshape. It is, of course, also possible that the second ring SR isconically in shape. FIGS. 8 and 9 show a cross-section along a line I-I,which is shown in each of the FIGS. 4 to 7. Also is shown that therotational axis RA may extend along a virtual line VL and that a sourceSO may be positioned such that it coincides with the line VL. In theembodiment shown in FIG. 8, the predetermined cross-sections may includefoils F extending from first ring FR to the second ring SR and beingconnected to at least first ring FR or second ring SR, as shown in FIG.8. However, as shown in FIG. 9, the predetermined cross-section may alsoinclude two sets of foils F1, F2 in one of the fashions shown in FIG. 4to FIG. 7. It also applies to at least one foil extending betweensupport S1 and support S2 that this foil coincides with a straightvirtual plane, that extends through the predetermined position, i.e. theposition which in use coincides with the source.

For the embodiment shown in FIGS. 8 and 9, it further holds that betweenthe foil and the predetermined position, in use, coinciding with asource SO, a tensed wire TW extends within the aforementioned straightvirtual plane. This means that radiation propagating from the source SOthat would hit the foil if the tensed wire TW were not present, will nowhit, and heat, the tensed wire TW, instead of that foil. As a result,the foil will be in the shadow of the tensed wire TW, will not absorb(EUV) radiation, and will consequently not be heating up due toabsorbance of (EUV) radiation. This significantly reduces thetemperature that the foil will reach under operational circumstances.

The tensed wire TW may be connected to the foil F. It may, for example,apply that a frontal part of a foil is effectively a tensed wire byproviding a row of perforations between that frontal part and aremaining part of the foil.

As shown in FIG. 8, it is possible that the tensed wire TW is held tightby a resilient element RE, such as a spring. It is possible that thetensed wire TW is thermally insulated from the foil F. The tensed wireTW may be made out of a material that includes tantalum and/or tungstenif the wire is not an integral part of the foil. In the embodiment shownin FIG. 8, the tensed wire TW extends fully along a diameter of thefirst ring FR. As shown in FIG. 9, it is possible that two tensed wiresTW are used. Each tensed wire TW may extend from a position on the firstsupport S1 to a closest position on the second support S2.

FIG. 10 schematically depicts a filter system FS for filtering debrisparticles out of the radiation beam. The filter system shown in FIG. 10is depicted as viewed from a predetermined position that is in useintended to substantially coincide with a source from which theradiation is generated. The filter system FS includes a plurality offoils F for trapping the debris particles. As will be clear later on,from this viewing position, the foils are seen as lines. FIG. 11 andFIG. 12 show one of these foils F, in, respectively, a perspective viewand view similar to that of FIG. 10. Each of the foils F includes twoparts FP1, FP2 that have a mutually different orientation. The two partsFP1, FP2 are connected to each other along a substantially straightconnection line CL, which is more clearly shown in FIG. 11. Each of thetwo parts FP1, FP2 coincide with a virtual plane (not shown) thatextends through the predetermined position from which the filter systemFS is seen in FIG. 10. This is schematically indicated by the virtualstraight lines VSL. As indicated earlier, this predetermined positionis, in use, intended to substantially coincide with a source SO fromwhich the radiation is generated. The source SO is schematicallyindicated in FIG. 11. The straight connection line CL also coincideswith a virtual straight line VSL that extends through the predeterminedposition, i.e. through the position that is intended to substantiallycoincide with the source SO from which the radiation is generated. Inuse, radiation, generated from the source SO, propagates through thefilter system. Only a small portion of the radiation will hit the foilsfrontally and may as such be absorbed by the foil, thereby resulting inheating the foil. Debris particles, traveling along a path into whichthe radiation propagates, may be trapped by the foils F as theirdirection of velocity is likely to have a component towards one of thefoils F. It is also possible to rotate the foil trap such that the foilsintercept the debris particles when these particles travel through thechannels C formed by the foils F. In addition to the absorbance ofradiation, the foils F also heat up, due to the impact of theseparticles.

The filter system FS includes a support S to which a first part FP1 ofthe two parts FP1, FP2 is connected at a first position P1 of thesupport S, a second part FP2 of the two parts FP1, FP2 being connectedat a second position P2 of the support S. In the embodiment shown inFIG. 10, the support S includes an inner ring IR and an outer ring OR.The inner ring IR and the outer ring OR are coaxial. A distance Dbetween the first position P1 and the second position P2 is fixed. Thefoils may be made of a material substantially including molybdenum.Also, the support S may be made of a material that substantiallyincludes molybdenum. The foils F may have been connected to the supportS by soldering.

The behavior of the foil trap shown in FIG. 10 when, in use, is asfollows. Each part FP1, FP2 of a foil F expands when heated up. Theexpansion occurs substantially within a plane in which the respectivepart lies. The expansion of the foil F is accommodated for by a movementof the connection line substantially sideways with respect to theoverall orientation of the foil. The extent to which the connection linemoves sideways when accommodating for the thermal expansion is even morepredictable when the distance between position P1 and position P2 isfixed. Foil F when heated up is in FIG. 12 schematically shown by adashed line.

A new orientation of the foil F, when heated up, has become predictabledue to the position of the connection line CL. As the straightconnection line coincides with a virtual straight line that extendsthrough the predetermined position which is, in use, intended tosubstantially coincide with a source SO from which the radiation isgenerated, and each of the two parts FP1, FP2 coincide with a virtualplane that extends through that predetermined position, a new positionand orientation of the foil will only cause a minimal drop in opticaltransmission, if at all. Furthermore, it is possible, for example, toexperimentally determine the thermal expansion and the new position of afoil when heated up, and to design the filter system such that when thefilter system is exposed to the absorbance of EUV radiation and/orimpact of debris particles, the foil adopts a orientation which allowsfor optimal transmission of (EUV) radiation.

Each part of the two parts FP1, FP2 may coincide with a virtual planethat is a straight plane. The controllability and predictability wouldthen be even more straight forward. However, it is possible that eachpart, or one of the parts FP1, FP2 includes a curvature. The embodimentshown in FIGS. 10 and 12 is focused on a cylindrical or conical filtersystem FS, i.e. a filter system having a cylindrical or conical outerring and possibly a cylindrical or conical inner ring. However, inprincipal, any other shape of the support and the filter system ispossible.

Fixation of the distance D between position P1 and position P2 of thesupport S is relative to the thermal expansion of the foil F. It is thuspossible that the distance D may slightly increase due to expansion ofthe support, i.e. in this case inner ring IR and outer ring OR.Schematically is shown in FIGS. 10 and 12 that a support may be cooledby a cooling system CS. For the sake of clarity, this cooling system CSis only shown to be present at the outer ring OR. It is, however,equally possible to provide cooling a system CS at the inner ring IR.The cooling system may be constructed as shown in FIG. 2 and FIG. 3.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in, for example, atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example, in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example, imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” as used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: an illumination systemconfigured to condition a radiation beam; a projection system configuredto project the radiation beam onto a substrate; and a filter system forfiltering debris particles out of the radiation beam, wherein the filtersystem comprises a plurality of foils for trapping the debris particles,a support for holding the plurality of foils, and a cooling systemhaving a surface that is arranged to be cooled, the cooling system andthe support being positioned with respect to each other such that a gapis formed between the surface of the cooling system and the support, andwherein the cooling system is arranged to inject gas into the gap.
 2. Alithographic apparatus according to claim 1, wherein a path between anentrance position at which the gas enters the gap and an exit positionfrom which the gas exits the gap forms a meandering path.
 3. Alithographic apparatus according to claim 1, wherein the gap is suchthat a smallest distance between the surface of the cooling system andthe support is in a range that varies from about 20 micrometers to about200 micrometers.
 4. A lithographic apparatus according to claim 3,wherein the gap is such that a smallest distance between the surface ofthe cooling system and the support is in a range that varies from about40 micrometers to about 100 micrometers.
 5. A lithographic apparatusaccording to claim 1, wherein the support is ring-shaped.
 6. Alithographic apparatus according to claim 5, wherein the support isrotatable around its axis.
 7. A lithographic apparatus according toclaim 1, wherein the surface of the cooling system is arranged to bestationary with respect to the support.
 8. A lithographic apparatusaccording to claim 5, wherein the surface of the cooling system issubstantially ring-shaped, sharing its axis with the support.
 9. Alithographic apparatus according to claim 1, wherein the surface of thecooling system is arranged to be cooled with a fluid.
 10. A lithographicapparatus according to claim 9, wherein the fluid is water.
 11. Alithographic apparatus according to claim 1, wherein the gas is argon.12. A lithographic apparatus according to claim 1, wherein the supportis provided with a recess for holding the gas before the gas exits thegap.
 13. A lithographic apparatus according to claim 1, wherein thecooling system is arranged to cool the gas before injecting the gas intothe gap.
 14. A lithographic apparatus according to claim 1, wherein atleast a part of the support and at least a part of the cooling systemform together a heat sink to which a number of first foils of theplurality of foils are connected, the first foils substantially freelyextending within the filter system such that heat is conductedsubstantially towards that heat sink through each first foil.
 15. Alithographic apparatus according to claim 14, wherein at least anotherpart of the support and at least another part of the cooling system formtogether an additional heat sink to which a number of second foils ofthe plurality of foils are connected, the second foils substantiallyfreely extending within the filter system such that heat is conductedsubstantially towards that additional heat sink through each secondfoil, wherein the filter system is arranged to filter debris particlesout of a predetermined cross-section of the radiation as emitted by asource, wherein the number of first foils extend substantially in afirst section of the predetermined cross-section, and wherein the numberof second foils extend substantially in a second section of thepredetermined cross-section, the first section and the second sectionbeing substantially non-overlapping.
 16. A lithographic apparatusaccording to claim 15, wherein at least one of the first foils and/or atleast one of the second foils is apart from its connection with therespective heat sink, and unconnected with respect to any other part ofthe filter system.
 17. A lithographic apparatus according to claim 15,wherein the filter system is arranged such that all of the filter systemremains below a predetermined maximum temperature when exposed to theradiation beam.
 18. A lithographic apparatus according to claim 15,wherein at least one of the first foils and at least one of the secondfoils extend in substantially the same virtual plane.
 19. A lithographicapparatus according to claim 18, wherein a distance in that virtualplane between the respective first foil and the respective second foilis selected so as to maintain a gap between that first foil and thatsecond foil when that first foil and that second foil reach theirrespective maximum temperatures.
 20. A lithographic apparatus accordingto claim 18, wherein that virtual plane extends through a predeterminedposition that is intended to coincide with a source from which theradiation is generated.
 21. A lithographic apparatus according to claim15, wherein at least one of the first foils extends between two of thesecond foils.
 22. A lithographic apparatus according to claim 1, whereinat least one foil of the plurality of foils comprises at least two partsthat are connected along a substantially straight connection line,wherein each of the two parts coincide with a virtual plane that extendsthrough a predetermined position that is intended to substantiallycoincide with a source from which the radiation is generated, thestraight connection line coinciding with a virtual straight line thatalso extends through the predetermined position.
 23. A lithographicapparatus according to claim 22, wherein a first part of the at leasttwo parts is connected at a first position of the support and a secondpart of the at least two parts is connected at a second position of thesupport.
 24. A lithographic apparatus according to claim 23, wherein adistance between the first and second position is fixed.
 25. Alithographic apparatus according to claim 22, wherein at least one ofthe at least two parts coincides with a virtual plane that is a straightplane.
 26. A lithographic apparatus according to claim 1, wherein atleast one foil of the plurality of foils substantially coincides with avirtual plane that extends through a predetermined position that is inuse intended to substantially coincide with a source from which theradiation is generated, and wherein a tensed wire extends within thevirtual plane between the at least one foil and the predeterminedposition.
 27. A lithographic apparatus according to claim 26, whereinthe tensed wire is connected to the at least one foil.
 28. Alithographic apparatus according to claim 26, wherein the tensed wire isheld tight by at least one spring element.
 29. A lithographic apparatusaccording to claim 26, wherein the tensed wire is thermally insulatedfrom the at least one foil.
 30. A lithographic apparatus according toclaim 26, wherein the tensed wire is made out of a material thatcomprises at least one of the metals of the group consisting of tantalumand tungsten.